1. Field of the Invention
The present invention relates to a process for the preparation of partially protected phenolic resins by acid-catalyzed reaction of a phenolic resin with an enol ether and subsequent treatment of the reaction mixture with a basic anion exchanger.
2. Brief Description of the Revelant Art
The production of highly integrated circuits currently requires the ability for structures having a width of less than 0.35 .mu. to be transferred imagewise to the substrate. For this purpose, use is predominantly made of chemically reinforced resist systems which generally comprise alkali-soluble binder resins whose alkali-solubilizing groups, for example, hydroxyl groups, have been blocked by acid-labile protecting groups, thereby rendering the binder resin substantially insoluble in alkali. Exposure then initates a primary reaction of an acid photogenerator which absorbs at the appropriate wavelength, forming a strong acid which, in the subsequent secondary reaction, results in elimination of the protecting groups and thus in re-formation of the solubilizing groups.
One example which may be mentioned of such systems is poly(p-hydroxystyrene) whose phenol groups are protected by, for example, tertbutyloxycarbonyloxy (TBOC) or tetrahydropyranyl (THP) groups (see for example: M. J. Bowden and S. R. Turner (eds.) "Electronic and Photonic Application of Polymers", ACS Series 218, Washington 1988; and N. Hayashi et al., Polymer 33, 1583 (1992)). These polymers, however, have disadvantages in respect of adhesion to silicon.
Attempts have already been made to improve adhesion, reproducibility and resolution by using in the resist formulations a binder resin whose phenolic hydroxyl groups have been replaced only partially by acid-cleavable protecting groups. One example of these attempts is EP-A 447,868, which proposes a radiation-sensitive mixture whose phenolic resin is partially protected by tetrahydropyranyl groups.
It is known that, in partially protected polymers having free phenolic monomer units, there is an increased occurrence of losses of thermal stability. The thermally induced decomposition can be attributed to partial deprotection which is caused in turn by the catalytic effect of the weakly acidic phenolic hydroxyl groups.
A partial protection provided by reacting the phenolic resin with the protecting-group reagent in the presence of catalytic quantities of acid is described, for example, in EP-A 447,868. The polymers prepared are isolated by introducing the polymer solutions obtained directly after the protection reaction into a precipitating agent.
The resists known to date have been unable to solve with satisfaction the problem of inadequate thermal stability. In systems containing very acid-labile protecting groups in particular, such as acetal and ketal protecting groups, the shelf life falls short of what is required. However, it is precisely acetal and ketal protecting groups which are preferred, since with the chemically reinforced resists they permit a greater processing scope than the less acid-labile protecting groups, for example the tert-butoxycarbonyloxy (TBOC) group or the ter-butyl ester protecting group.
The object of the present invention is to develop a process for the preparation of heat-resistant polymers and to provide positive-working, highly active, radiation-sensitive systems, in particular for the production of relief structures, which systems do not have the abovementioned disadvantages; in other words, they should in particular possess good adhesion and processing stability, be sensitive to UV radiation, electron beams and X-rays and should in particular possess good storage stability (shelf life) and permit high resolution.
WO 94/14858 describes a positive photoresist composition based on poly(hydroxystyrene), whose content of metal ions and chloride ions has been drastically reduced by purification with an anion exchanger.